This paper will given an overview of the current state of the art of polymer light-emitting devices as they are manufactured at Philips. Electro-optical characterization as well as lifetime measurements will be discussed, with emphasis on the use of electroluminescent polymer materials and processes in monochrome graphic displays. The factors that determine power consumption on a system level will be detailed. Initial results on inkjet-printed devices will be shown.

New device performance data are reported for red, green, blue and white polymer light emitting diodes (PLED). The devices all show CIE color co-ordinates approaching the PAL standards. In this report we also discuss the efficiency and lifetime data for these test devices and show that these systems could be used as components in full/multi color high information content (HIC) displays.

Organic LED (OLED) technology promises superior performance in brightness and color resolution, wider viewing angles, lower power consumption, thin displays, and robust physical characteristics. These advantages make OLED displays attractive for next-generation flat panel displays. This paper describes the state-of-the-art in OLED technology and addresses some of the benefits and difficulties facing the integration of OLED devices into a range of imaging equipment applications. We will review OLED performance from a systems perspective and will compare it to OLED material and panel properties. We will also describe the competitive attributes of a flat panel display and recent work done at Kodak on interfacing to OLED devices.

We have developed highly efficient, top-emitting white OLED structures suitable for black-and-white or full color microdisplay applications. White light emission was obtained from both singly doped and doubly doped emitter layer structures. However, the double-dopant structure, employing fluorescent blue and red-emitting dyes, generated much higher luminous efficiency than that of the single dopant structure incorporating only the red dopant. For top- emitting, white OLED devices with the double-dopant structure, we have achieved luminous efficiencies of 974 cd/m², 4.9 cd/A, 1.9 lm/W and an external quantum efficiency of 1.8% when driven at 20 mA/cm² and 7.9 V. The device has an electroluminescence turn-on voltage of 2.2 V. After turn-on, the current varies as the 14th power of the voltage. The projected operational half-life of these devices is greater than 6000 hrs. This estimate is based on devices that have been continuously driven at a constant current of 20 mA/cm² for more than 3000 hrs. Based on an up-emitting, double-dopant white OLED structure, we have developed an SXGA-resolution, black-and-white active matrix OLED (AMOLED) on silicon microdisplay. This AMOLED-on-Si has demonstrated real-time video with 256 gray levels and consumes less than 400 mW of power at a brightness of 200 cd/m².

Capable self-emissive polymers are being developed for use as emitting materials for a variety of display applications. This paper describes the use of standard CMOS integrated circuit silicon wafer technology along with a spin-cast polyfluorene-base polymer emissive layer, to demonstrate an XGA resolution, full video microdisplay. The silicon chip drive circuitry (Analog Pixel-APIX) is described along with results from our efforts to optimize the reflective anode, the semitransparent cathode process, and emissive cell construction. The 1024 X 768 pixel display achieves 200 Cd/m2 brightness at low power (<50 mW) with fast 1 usec response times. In addition, we summarize future directions to achieve color and the need to incorporate a production- worthy seal layer on microdisplays manufactured on silicon wafers.

The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene] (BEH-PPV) 4 was prepared by Gilch polycondensation, and its optical properties were compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) 1. The precursors for the Gilch method have traditionally been prepared by methods which have certain disadvantages. These can be overcome by the use of directed metallation reactions which are illustrated in the synthesis of some poly(2,5-disilyl-substituted 1,4-arylene vinylene) derivatives.

A method of large-area dye diffusion for patterning the emissive color of organic light-emitting devices (OLEDs) using transferred photoresist is introduced. Using a large-area dye diffusion source, the dye is diffused into the emissive layer of the OLED. To locally pattern the emissive color, either a shadow mask or a patterned dye source has to be used. The implementation of both approaches using photolithographic patterning and transferred photoresist is outlined in this paper. In addition, a method using transferred photoresist for patterning of the cathode will be discussed.

We have introduced acene such as anthracene, naphthacene and pentacene into the main chain of polyfluorene to improve stability and to realize color tuning to pure blue, green and red. High molecular weight acene-containing polyfluorenes are successfully synthesized by the Ni[0] mediated Yamamoto-coupling reaction. Average molecular weights of the copolymers are Mw equals 42000 - 96000 (determined by GPC). By the introduction of 10% of acene moiety, the device stability could be significantly improved, indicating the torsion between the acene and fluorene units suppresses interchain aggregation; the copolymers exhibits stable blue, green and red emission in the anthracene-, naphthacene- and pentacene-containing polyfluorenes, respectively. Onset voltage for light emission is 5 - 30 V and the maximum luminance is 240 - 5200 cd/m². As a side effect an unique color change on UV irradiation could be observed as well as the reason for its appearance determined.

We describe a flexible, transparent plastic substrate for OLED display applications. A flexible, composite thin film barrier is deposited under vacuum onto commercially available polymers, restricting moisture and oxygen permeation rates to undetectable levels using conventional permeation test equipment. The barrier is deposited under vacuum in a process compatible with conventional roll- coating technology. The film is capped with a thin film of transparent conductive oxide yielding an engineered substrate (Barix^TM) for next generation, rugged, lightweight or flexible OLED displays. Preliminary tests indicate that the substrate is sufficiently impermeable to moisture and oxygen for application to moisture-sensitive display applications, such as organic light emitting displays, and is stable in pure oxygen to 200 degrees Celsius.

The importance of interfacial properties in organic light emitting devices (OLED) is well recognized. Using mainly surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment, we have studied the formation of interfaces that occurs in OLEDs using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The results have shown that the interface energy level alignment is intimately related to the charge injection and therefore to the device characteristics and performance. Relationship to interface charge transfer, chemical reaction, and dipole layer formation will also be discussed.

An appropriate choice of the cathode material and the process of cathode deposition is a key issue in the development of polymer light emitting devices. In this paper, we report on the impact of low work function metals on the luminescence efficiency of thin films of polyfluorene type polymers. Photoluminescence as well as electroluminescence experiments are presented, and in both cases, a strong correlation between the metal layer thickness and the luminescence efficiency is demonstrated. By means of time-of-flight secondary ion mass spectroscopy (TOF-SIMS), the distribution of the metal contamination within the polymer layers is determined. The results strongly suggest that impurity quenching of excitons by metal atoms inside the polymer layer takes place and strongly affects luminescence and device efficiency.

We have measured the electrical characteristics at the contact between Indium Tin Oxide (ITO) and N-N'-diphenyl-N- N'-bis(3-methylphenyl)-1-1-biphenyl-4,4'-diamine (TPD). By carrying out independent measurements of the hole mobility in the organic layer we are able to isolate the injection and the transport processes and study the electrical characteristics of the contact alone. We find that the contact is in fact not Ohmic, but current-limiting, despite the fact that the current has a similar electric field dependence as the space charge limited current. The contact remains current-limiting even when the hole mobility is lowered by three orders of magnitude by diluting TPD into polycarbonate. The data support a model of injection in disordered materials.

The band alignment at the interface between electroluminescent polymers and the electrodes in polymer- LEDs was studied using photoelectron spectroscopy. Chemical factors like the formation of InCl₃ during conversion of precursor-PPV on ITO could be directly monitored with XPS. Films of electroluminescent polymers were studied on a range of ITO and metal electrodes with different work functions, as well as with an intermediate, electrically conducting polymer layer, using UPS. For the polymers spin-coated directly onto the substrates, the vacuum levels are aligned. In the case of conducting polymer films on ITO or metal substrates, the F¹ermi levels are aligned. With a conducting polymer layer sandwiched between the electroluminescent polymer and the ITO electrode, the polymer bands align to the vacuum level of the conducting polymer. Thus, the barrier to hole injection into the electroluminescent polymer is determined by the work function of the conducting polymer instead of the work function of the ITO electrode. Furthermore, the study of the band alignment at polymer electrode interfaces was extended to three-layer structures: The results are in agreement with the common assumption that the potential drop over the polymer layer in a polymer LED is equal to the difference between the electrode work functions.

We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. Using phosphorescence based emitters we have eliminated the inherent 25% upper limit on emission observed for traditional fluorescence based systems. The quantum efficiencies of these devices are quite good, with measured external efficiencies > 15% and > 40 lum/W (green) in the best devices. The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Forster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. We discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations. Accelerated aging studies, on packaged devices, have shown that phosphorescence based OLEDs can have very long device lifetimes.

The temperature stability of white and blue OLEDs was investigated by observing the I-V, EL-V and the spectral characteristics of various devices stored at elevated temperature (up to 130 degrees Celsius). Blue multilayer organic light emitting diodes (OLEDs) containing PEDT (polyethylenedioxythiophene) or PANI (polyaniline) derivatives as the hole injection and puffer layer, aromatic diamines like Spiro-TAD (2,2',7,7'- tetrakis(diphenylamino)spiro-9,9'-bifluorene) as a hole transport material (HTM), Spiro-DPVBi (2,2',7,7'- tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene) as an emitting material (EM) and of Alq₃ (tris(8-hydroxy- quinoline)aluminum) as the electron-injection and electron- transport layer (ETL) were fabricated. White OLEDs were prepared, containing an additional DCM (dicyanmethylene-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran) doped Alq₃ layer between the Spiro-DPVBi and Alq₃ layer. Use of Spiro-TAD as a hole transport material (HTM) and of Spiro- DPVBi as an emitting material (EM) resulted in dramatically improved temperature stability: for the white and blue OLED no significant deterioration up to 130 degrees Celsius were found. Devices consisting of non spiro components like NPB and/or DPVBi already started to degrade at much lower temperatures.

We have developed a new type of polymers having TPD unit combined with non-conjugated spacer group, poly(arylene ether sulfone)-containing and poly(arylene ether ketone)- containing tetraphenylbenzidine (PTPDES an PTPDEK) and also polymers with directly coupled triphenyl amine units (PPBA). When these polymers are mixed with strong acceptor such as TBPAH or DDQ, they indicated higher conductivity and facilitated hole injection from ITO to the hole transport layer. Spin-coating of such polymer from an organic solution on ITO was found to improve the surface roughness of ITO, resulting in reduced defects that cause electric short circuit between ITO and cathode. These buffer materials lowered the operation voltage and improved the thermal stability of the device. After storage of 1,000 hours at 85 degrees Celsius, the device with polymer buffer showed no degradation in luminance and small increase of operation voltage. In comparison with CuPc buffer, it is clear that the doped polymer is superior in terms of both efficiency and thermal stability.

Organic light-emitting devices (OLEDs), made of polymer bi- layer thin films, on the thin flexible plastic substrates have been fabricated and studied. The ITO and Ca/Al electrodes have been used as anode and cathode, respectively, in this structure. Both polymers -- the hole- transporting and electron-transporting/light emissive co- polymers -- used in this study demonstrated a very good thermal and photo-oxidative stabilities. The hole transporting co-polymer (amine-fluorene) thin film is amorphous, while the light emissive/electron-transporting co-polymer (benzothiadiazole-fluorene) thin film is partially crystalline. The photoluminescence (PL) and electroluminescence (EL) emission peaks are both located around 550 nm. The plastic substrate coated with ITO used in this study is less than 0.2 mm thick and highly flexible. It has a sheet resistance in the range of about 10 to 20 (Omega) /D'Alembertian, and the transmittance higher than 80% in the photon energy ranging from 470 to 750 nm; the maximum transmittance is approximately 84.5%. The root-mean-square (RMS) of the ITO surface roughness is approximately 1.70 nm. The oxygen and water vapor transmission rate through the flexible plastic substrate was about 14.2 X 10^-6 cc/cm²(DOT)day(DOT)atm and 1.14 X 10^-4 gm/cm²-day-atm, respectively. For the OLEDs fabricated on the thin flexible plastic substrates, the typical turn-on voltage is less than 6 V, and maximum brightness is higher than 13,000 cd/m². The maximum emission efficiency, luminous efficiency, and external quantum efficiency for our OLEDs are 66.2 cd/A, 18.3 lm/W, and 16.2%, respectively.

Thin buffer layers composed of tris(8-quinolato) aluminum (Alq) layer doped with Li-acetylacetonate (Li-acac), Mg-acac and magnesium metal were inserted between an MgAg cathode and an Alq electron transport layer in standard double-layer devices with the ITO/TPD/Alq/MgAg structure. The insertion of the buffer layers with Li-acac brought about large decrease in drive voltage. While the effects of Mg-acac and magnesium metal were smaller than the case of Li-acac. Electron drift mobility of Alq layers doped with Li-acac and magnesium metal was evaluated by using a time-of-flight method. Large increases of electron mobility as well as the increase of dark conductivity with doping were found.

New hole-transporting amorphous molecular materials with high glass-transition temperatures for organic electroluminescent devices have been developed and their charge-transport properties investigated. They comprise 4,4',4'-tris[biphenyl-2-yl-(phenyl) amino]triphenylamine, 4,4'4'-tris[biphenyl-3-yl- (phenyl)amino]triphenylamine, and 4,4'4'-tris- [biphenyl-4-yl-(3-methylphenyl)amino]triphenylamine, 4,4'4'-tris[9,9-dimethyl-2- fluorenyl(phenyl)amino]triphenylamine, N,N'-di(biphenyl- 2-yl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine, N,N'-di(biphenyl-3-yl)-N,N'-diphenyl-[1,1'-biphenyl]- 4,4'-diamine, N,N'-di(biphenyl-4-yl)-N,N'-diphenyl-[1,1'- biphenyl]-4,4'-diamine, N,N'-bis(9,9-dimethyl-2- fluorenyl)-N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine, and N,N,N',N'-tetrakis(9,9-dimethyl-2-fluorenyl)-[1,1'- biphenyl]-4,4'-diamine. Hole drift mobilities of these materials were determined by the time-of-flight method, and the electric field and temperature dependencies of drift mobility were analyzed in terms of the disorder formalism.

The influence of interfacial charges on the device characteristics of multilayer organic light-emitting diodes (OLEDs) is investigated, and a concept to improve device performance is presented. We studied devices consisting of copper phthalocyanine (CuPc) as hole injection and buffer layer, N, N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as hole transport layer, and tris(8- hydroxyquinolinato)aluminum (Alq₃) as electron transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium electrode. Detailed current-voltage measurements show that the device characteristics in negative bias direction and at low positive bias below the built-in voltage depend strongly on the bias sweep direction, indicating that interfacial charges have a pronounced influence on the device characteristics. Low-frequency capacitance-voltage experiments reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, evidence of a redistribution of the internal electric field in this device configuration. Time-resolved electroluminescence (EL) measurements proved that also the EL response time at low voltages is governed by the accumulation of charge carriers inside the device rather than by their transport. Optimizing the device structure by grading the organic-organic interfaces results in an enhanced current flow, an improved brightness, and a faster EL response time. Our investigations clearly indicate that the abrupt CuPc-NPB as well as the NPB-Alq₃ interface significantly influence the performance of our multilayer OLED.

Multilayer organic light-emitting device with phosphorescent guest emitter, tris(2-phenylpyridine)iridium [Ir(ppy)₃] doped in a host 4,4'-N,N'-dicarbazol-biphenyl (CBP) layer, exhibited very high luminous efficiency. A device having such phosphorescent guest emitter could not offer sufficiently long driving lifetime required by real products. On the other hand, phosphorescence in organic molecules rarely occurs at room temperature. We studied ways to increase driving lifetime by three types of experiments. First, we investigated to see whether the driving lifetime is dependent on guest molecule concentration. Second, we inserted Copper phtalocyanine (CuPc) between Indium Tin Oxide (ITO) anode and hole transport layer, 4,4'-bis[N- (naphthyl)-N-phenyl-amino]biphenyl (NPB) to prevent driving voltage from rising during constant current operation. Finally, we exchanged the hole blocking layer, which is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), for [(1,1'-biphenyl)-4-olato]bis(2-methyl-8- quinolinolato N1,08)aluminum (BAlq). Optimizing all of the above mentioned steps, we can expect the half decay lifetime of 20000 hours or over at an initial luminance of 100 cd/m² by constant current driving.

We synthesized lithium quinolate complexes, 8- hydroxyquinolinolato lithium (Liq) and 2-methyl-8- hydroxyquinolinolato lithium (LiMeq) as emitter and electron injection/transport materials to be used in conventional two layer organic light emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenyl benzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8- hydroxyquinolinolato Al(III) as emitter material. The device efficiency using these complexes were optimized using combinatorial methods. We also compared the electron injection, transport and emission properties of Li-complexes with the well known emitter Alq₃ in the same experiment taking advantage of the combinatorial approach. The Li- quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq₃ devices. Contrary to the Alq₃ emission, the Li-quinolates exhibit bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence for exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li-quinolate and DMeOTPD with the observed EL spectrum. However, the lithium complexes increase the efficiency of an optimized ITO/DMeOTPD/Alq₃/Al device considerably when used as a thin interface layer between Alq₃ and aluminum. The improvement of device characteristics using lithium quinolates is similar to that obtained using LiF salt. The mechanism of improvement of efficiency using a lithium complex interface layer seems to be of chemical doping nature.

In recent years, considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, matrix displays consisting of organic light emitting diodes (OLEDs) have been brought into marketable commodity. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. In contrast to polymer devices, OLEDs are processed in ultrahigh vacuum systems. The organic source materials are sublimated from effusion cells. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal-organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode setups. Further insight in the energy band alignment at organic heterointerfaces are gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical (I-V, C-V) and optical (P-I) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.

A two-step kinetic model for the recombination process is presented to describe experimentally observed emission characteristics of organic light-emitting diodes (LEDs) including their electric field-dependent quantum efficiency (QE) and field evolution of the electroluminescence (EL) spectra. The model assumes the emitting excited states to be produced through an intermediate charge pair (CP) state whose decay is subject to the competitive processes of which formation of various emissive states, such as localized molecular excitons, locally excited excimers and exciplexes, or electroplexes, and dissociation into two separated carriers are expected to be of crucial importance for the QE of organic LEDs and shape of their EL spectra. Depending on the carrier injection efficiency at the electrodes and the magnitude of applied field the recombination probability can be either governed by the CP formation-to-transmit time ratio (usually at low fields) or the capture-to-dissociation time ratio of CP states (usually at high fields). Combining both an expression for the QE in organic LEDs is proposed. The field and temperature dependence of the QE is discussed, showing that the reciprocal of QE can be approximated by a Poole-Frenkel-type field dependence with a negative gradient at low fields and positive gradient in the upper limit of the accessible field range. The experimental data for single layer (SL) and double layer (DL) LEDs confirm these predictions; as a rule, the QE increases in the low-field, and decreases in the high-field regime of applied fields. Increasing probability of CP dissociation, considered as a part of the overall recombination process, is thus a limiting factor for the QE, unless the increasing field enhances in some way formation of emitting states. Such an effect is observed with some organic DL LEDs, where donor- acceptor species form heteromolecular CPs with their electric dipole moments oriented antiparallel to the applied field.

If conduction in a polymer light emitting diode is by trap- free, space-charge-limited current (TFSCLC), the injecting contact must be ohmic. Such an ohmic contact is, by definition, the perfect injecting electrode. The absolute injection efficiency of any specific contact can therefore be quantified by the ratio of the actual current density, J, to that of the ideal TFSCLC current density, J_TFSCLC. In this work we report on the injection efficiency of positive carriers from pretreated indium tin oxide (ITO) and metal electrodes into poly(9,9-dioctylfluorene) (PFO) and two representative copolymers. The ITO was either washed only in solvents, exposed to an oxygen plasma or coated in a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid blend. Time-of-flight photocurrent measurements were used to find the positive carrier mobility, (mu) _p, so that J_TFSCLC could be calculated. Dark injection current transient measurements were used to confirm the presence or absence of TFSCLC conduction. Steady-state, current density- voltage measurements were then used to compare the actual current to J_TFSCLC, thus allowing the absolute injection efficiency, J/J(subscript TFSCLC, to be calculated.

Nanosecond electroluminescence (EL) spikes observed at the voltage turn-off when multilayer blue 4,4'-bis(2,2'diphenyl vinyl) -1,1'-biphenyl (DPVBi)-based organic light-emitting devices (OLED's) are excited by rectangular voltage pulses are described. The spikes exceed the cw brightness by up to an order of magnitude. Time-resolved images of the devices demonstrate that the emission from most of the sample surface decays with a single time constant (tau) ₁ equals 13 +/- 3 ns. This decay is attributed to recombination of charges which accumulate at the interface of the electron and hole transporting layers, possibly at intrinsic trapping sites. In areas of increased electron injection and EL such as cathode edges and morphological defects, a second slower decay time 20 ns < (tau) ₂ < 1 microsecond is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in (tau) ₁ are found between bright and dim areas of the devices. At a bias of 10 V, the amplitude of the spike is found to peak at a pulse duration of approximately 20 microseconds. It is noted that similar OLED's, in which the DPVBi was replaced by tris-(8- hydroxyquinoline Al) (Alq₃), did not exhibit such spikes.

Recent discoveries of degradation mechanisms in organic light emitting diodes (OLEDs) are reviewed. One common observation regarding the OLED reliability is the gradual increase of the diode driving voltage under forward bias. This trend reverses when the bias polarity is reversed, and the initial driving voltage can be recovered almost completely. The mobile ions are exposed to explain the phenomenon. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Another advancement in the field is the utilization of a single organic alloy layer to improve the device lifetime. A typical organic alloy layer consists of a mixture of hole and electron transport materials. Significant lifetime improvement over heterojunction structures is achieved with comparable efficiency. The significant improvement in the device reliability is attributed to the elimination of the heterojunction interface and the minimization of the formation of Alq₃ cations. The high efficiency is achieved due to the balanced carrier injection, transport and confined recombination in the device.

Conduction in aluminum(III) 8-hydroxyquinoline (Alq₃)- based organic light-emitting diodes (OLEDs) was modeled based on trapped charge-limited conduction of electrons in the Alq₃ bulk. This model was chosen since it can easily incorporate an arbitrary trap distribution such as may arise during the material's degradation. The evolution of a narrow Gaussian distribution of localized trap states below the lowest unoccupied molecular orbital (LUMO) of Alq₃, lying against a natural exponential background, was used to explain changes in the current-voltage characteristic and external quantum efficiency with time observed by many researchers for organic light-emitting diodes. Based on the change of the shape of the DC current density vs. voltage (J-V) curve, the depth of the electron trap states that were formed during aging was about 0.25 eV below the LUMO of Alq₃. This value is consistent with electrochemical measurements of known chemical degradation products. The J-V characteristics show behavior which is suggestive of a trap- filled limit, and this is discussed along with the general appropriateness of the model used.

Poor device stability has been a major concern for organic light emitting devices (OLEDs). The relatively short operational lifetime of the OLEDs is predominantly attributed to an intrinsic degradation behavior, which leads to a decrease in the electroluminescence quantum efficiency of the devices in time. Recently, we found that the injection of holes in tris(8-hydroxyquinoline) aluminum (AlQ₃), the most widely used organic electroluminescent material, is the main factor responsible for the intrinsic degradation behavior in OLEDs. The photoluminescence quantum efficiency of AlQ₃ has been found to decrease as a result of predominantly hole current flow. Further studies using time-resolved fluorescence measurements reveal that degradation is also associated with a decrease in the lifetime of the AlQ₃ excited states, thus revealing the nature of the degradation products as luminescence quenchers. Various phenomena pertaining to device degradation will be discussed.

The methods of thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) were employed to reveal the trap structure of the most prominent organic semiconductors materials such as tris-8-(hydroxyquinoline) (Alq₃), N-N'-di(1-naphtyl)-N-N'-diphenylbenzidine ((alpha) -NPD), and 4,4',4'-tris-(N-2-naphtyl)-N-phenylamino- triphenylamine (1-Naph-DATA). The energetic trap depths and a lower limit of the trap densities were derived for all investigated materials by means of the initial-rise method and curve fitting techniques. Typical activation energies range between 0.1 and 0.6 eV and trap concentrations differ between 10¹⁴ and 10¹⁷ cm^-3. Most materials exhibit trap levels with a single activation energy, however, in Alq₃ a brought distribution of trap depths will be reported. In addition, the polarity of the dominant trap levels was determined by a comparison of TSC spectra from optically and electrically filled traps. Besides the trap detection and characterization the effect of doping and accelerated aging on the trap structure will be shown. TSC and TSL results on rubrene doped Alq₃ reveals a characteristic shift in the trap depth indicating new rubrene related trapping site. The effect of aging on the trap structure of organic semiconductors in 'potentially harmful' atmospheres such as oxygen and humidity and their correlation to I-V characteristics will also be reported.

Electroluminescent diodes fabricated on silicon substrates which emit at a wavelength of 1.5 micrometer have been demonstrated. The diodes operate at room temperature and exhibit good I-V characteristics. The diodes use an erbium tris(8-hydroxyquinoline) (ErQ) layer as electron transporting and emitting layer and use N, N'-diphenyl-N, N'-bis(3-methyl)-1,1'-biphenyl-4,4'-diamine (TPD) as the hole transporting layer. Hole injection into the diodes is from a p⁺⁺ silicon substrate anode and aluminum is used as the cathode electrode. The devices demonstrated start to exhibit electroluminescence at a voltage of approximately 17 V and the electroluminescence intensity rises sub-linearly with the current density through the device. At a drive voltage of 33 V the diodes have an internal efficiency of approximately 0.01%. We have measured the luminescence lifetime for the 1.5 micrometer emission and obtained a value of approximately 200 microsecond(s) . Using this value and estimating the total concentration of erbium present in the diodes we calculate a theoretical maximum optical power generation in these diodes of approximately 100 mW.

We present a new class of thiophene-based molecular materials with high solid-state photoluminescence efficiencies across the entire visible range. We show that full color tunability may be achieved, including white and purple, by binary blends of these materials, according to CIE standards. With some of these materials efficient electroluminescent devices were fabricated and characterized.

Three kinds of distyrylbenzene derivatives (DSB-1 -3 and -5) which are trimers of p-phenylenevinylene unit were vapor- deposited on a highly oriented PPV friction-transferred layer. Their structural and optical properties in thin film were investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and ultraviolet-visible (UV- VIS) spectroscopy. DSB derivatives grew pseudo- homoepitaxially on a PPV friction-transferred layer. The molecular axes of DSB-1 and -3 were oriented parallel to the sliding direction of PPV. On the other hand, DSB-5 molecules adsorbed obliquely to the substrate surface. The polarized absorption spectra of DSB-1 and -3 films indicated remarkable anisotropic features. The dichroic ratios of absorption intensity (I_(parallel)I_{(perpendicular})) were estimated to be 10 and 12 for DSB-1 and -3 films, respectively. The enhanced anisotropic properties were discussed based on the molecular orientation in the film.

We present a complete characterization of Organic Light Emitting Diode (OLED) structures performed in an ultra-pure Ultra High Vacuum (UHV) environment and under controlled influence of oxygen and atmospheric gases. We fabricated and characterized standard NPB/Alq₃ devices with an Indium Tin Oxide (ITO) anode and a magnesium cathode in an UHV system. With this system we are able to study the injection properties of very clean, controllable interfaces in the absence of any impurity gas. We found that the threshold voltage for OLED operation always increased after exposure to any atmospheric gas, an indication of deteriorated injection properties. However, the luminescence efficiency can become higher after exposure to impurity gases. Without contact to air the OLED do not degrade with appearance of so called 'black spots.' To investigate the intrinsic stability of the OLEDs in ultra-high vacuum we performed a realtime observation on the surface of a 35 nm thin magnesium cathode with Ultraviolet Photoelectron Spectroscopy (UPS). We found that even with a 35 nm thin magnesium-cathode, the underlying organic layer never appeared at the surface also after hours of operation in the ultra-pure conditions. The only sign of deterioration at the cathode is a slow oxidation of the magnesium surface. Thus, OLEDs with semitransparent cathodes are stable if the are operated under ultra-high vacuum conditions.

Alq₃-based organic light emitting diodes (OLEDs) were prepared by molecular beam deposition in Ultra-High Vacuum (UHV) and their electrical properties were studied by impedance spectroscopy before they were exposed to any impurity gases. To characterize the fundamental injection processes, we studied the simplest OLED structure, consisting of a single layer of Alq₃ between an ITO anode and a Mg cathode. In UHV, and below the threshold voltage for luminescent, instabilities in the frequency dependence of the resistivity, as well as in the current vs. voltage characteristics, are observed, which could be related to inhomogeneous contacting at the electrodes. When the same experiment is performed in air, both kinds of instabilities disappear, which demonstrates that exposure to atmospheric gas plays a role in stabilizing the contact and injection properties in the devices. Additionally, the impedance spectrum in air below the threshold voltage for luminescent shows an additional feature that hints to two different regions in the Alq₃ layer, which we assign to the effect of Mg atoms which diffused into the Alq₃ layer during deposition.

Organic bilayer light emitting devices have been constructed using rubrene-doped poly(2-methoxy-5(2'-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) as the emissive material and a novel hybrid compound consisting of pyridine and oxadiazole -- 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5- yl]pyridine (PDPyDP) -- as an electron transport layer. Although such structures exhibited a higher external quantum efficiency than those in which the electron transport layer was 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5- yl]benzene (OXD-7) they degraded relatively quickly, both in storage and under an electrical stress. Considerable improvements in the device lifetimes were obtained by the insertion of a copper phthalocyanine layer between the anode and the MEH-PPV layer and a tris (8-hydroxyquinoline) aluminum (Alq₃) layer between the PDPyDP layer and the Al electrode.

We have fabricated highly efficient organic light-emitting diodes (OLEDs) using novel hole-transporting emissive materials with triphenylamine moiety. The novel emissive materials have a high glass transition temperature ranging from 141 - 152 degrees Celsius, which is attributed to nonplanar molecular structure. The OLEDs consist of an emitting layer of the novel emissive material and an electron-transport layer of tris(8-qunolinolato) aluminum (Alq₃). Emission colors of the OLEDs were bluish-green and greenish-yellow. High external quantum efficiency of 1.2 - 2% was obtained at a luminance of 300 cd/m², and good durability in a continuous operation at room temperature and high temperatures was achieved.

The poly[2-(9',9'-bishexylfluorenyl)-1,4- phenylenevinylene] (BHF-PPV), which is PPV containing 9,9- bishexylfluorene as a pendant group, was synthesized by the modified Gilch dehydrohalogenation polymerization of the corresponding bischloromethyl-substituted benzene monomer. The energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of BHF-PPV were 5.35 and 2.94 eV as determined by cyclic voltammetry. Band gap, estimated from both cyclic voltammetry and optical absorption measurement, agrees well to be 2.41 eV. The EL spectrum showed two peaks at 504 and 535 nm, which very closely resembled the PL spectrum of the polymer film, demonstrating that the PL and EL originate from the same excited state. Blue-green LED was fabricated using BHF-PPV as the emissive layer, PEDOT:PSS as the hole-injection layer, and Mg-Ag alloy as the cathode. The device emitted bright blue-green light with turn-on voltage of 3.0 V and exhibited luminance efficiency and power efficiency of 0.64 cd/A and 0.45 lm/W, respectively, at the luminance of 105.1 cd/m² driven at the voltage of 4.5 V and current density of 16.37 mA/cm².

Self-organizing layered perovskite compounds like (formula available in paper) naturally form a dielectric quantum-well structure in which semiconducting PbI₄ layers and organic (C₆H₅C₂H₄NH₃) layers are alternately piled up. Due to their low- dimensional semiconductor nature they exhibit a strong absorption and sharp photoluminescence from the exciton band. In electroluminescent devices pure green emission peaking at 520 nm with a very narrow half-width of about 10 nm has been reported. As the organic-inorganic layered structure has promising properties for EL-devices, we investigated two- and three layer structures using this perovskite as emitter material in combination with additional hole and electron injection layers. To get more insight into electrical properties and electroluminescence- mechanisms of this material, temperature dependent current- voltage-luminance characteristics have been measured, showing an increasing onset-voltage for current flow from 2.6 V at room temperature to about 8.8 V at 80 K. Electroluminescence is detected at temperatures below 150 K with onset voltages of about 13 V. At liquid nitrogen temperature efficiencies of (formula available in paper) are obtained.

In this paper the monodomain-alignment of new photocrosslinkable fluorene bisacrylate model compounds and oligomers is reported. The orientation of the nematic LC- phase is preserved by photopolymerization in an oriented LC- network. Monodomain orientations of crosslinked fluorene bisacrylates are investigated by UV-VIS and photoluminescence spectroscopy comparing different alignment layers. OLEDs were fabricated using oriented fluorene bisacrylates and polyfluorenes leading to state-of-the-art polarization ratios for electroluminescence of up to 25 and brightnesses of about 250 Cd/m².

Bipolar tetraphenylmethane derivatives carrying both hole transporting and electron transporting units were synthesized and characterized. Tetraphenylmethane core structure of the molecule provides an effective molecular skeleton for inhibiting the crystallization of the attached functional moieties and greatly enhances the glass-forming ability of the molecules. A single-layer light-emitting device was fabricated based on the highly luminescent bipolar tetraphenylmethane-based derivatives containing conjugated triphenylamino and 2,5 diaryl substituted 1,3,4- oxadiazole moieties. The current-voltage-electroluminescence characteristics of the devices are described.

Organic polymer light-emitting devices (Poly OLEDs) with the bi-layer thin film structures have been fabricated on both glass and flexible plastic substrates. The structural, optical, and electrical properties of each organic polymer layer have been optimized for the Poly OLED optimum electrical performances. The spin coating technique was used to fabricate our devices. An alternating co-polymer of poly (9, 9'-dihexyl fluorene-2, 7-diyl) and poly (benzothiadiazole 2, 5-diyl) (PHF-BTD), and poly (9-hexyl carbazole-3, 6-diyl) (PC6) have been used as light emitting and hole transporting polymer, respectively. The combination of the optical and cyclic voltammetry (CV) and analysis has been used to develop the band diagram for our Poly OLEDs. For our polymers, the oxidation/reduction processes are well defined and clearly observed in the CV spectra. This allows a direct comparison of the polymers band gaps determined by both the optical and electrical methods. The photoluminescence and electroluminescence emission peaks are both located around 570 nm, and maximum brightness up to 2000 cd/m² was obtained. Turn-on voltages of approximately 16 and approximately 20 V, acceptable emission efficiencies of approximately 5.5 and approximately 4.2 cd/A, good power efficiencies of approximately 0.9 and 0.6 lm/W, and acceptable external quantum efficiencies of 1.8 and 1.6% have been obtained for OLEDs fabricated on both glass and plastic substrates, respectively.

We present electroluminescence (EL) studies of bilayer organic light-emitting diodes formed from poly(N- vinylcarbazole) (PVK) and the vacuum-deposited poly(p- phenylene) (PPP). The ITO/PPP/Al devices show EL spectra with a blue emission peak around 450 nm, very similar with the photoluminescence (PL) spectra of PPP. However, the EL spectra of the ITO/PVK/PPP/Al devices show a broad shoulder around 550 nm, which is dependent upon the bias voltage, as well as the peaks at the shorter-wavelength region originating from the PPP layer. The shoulder around 550 nm becomes more pronounced at the low bias voltage while the EL peaks originating from PPP become stronger with increasing bias voltages. This behavior can be accounted by exciplex formation at the interface between PVK and PPP. The current- voltage-luminescence characteristics and the quantum efficiency of both devices are almost independent of temperature in the temperature range between 15 and 300 K, indicating a tunneling mechanism for charge carrier injection.

In this paper, we report an effective thermal transfer process for performing controllable and selective doping of polymer films for multicolor organic light-emitting devices (OLEDs) or color pixels in OLED displays. In this process, the polymer receiver film is placed in direct contact with the dye-dispersed polymer donor film (with patterns) to permit direct dye-diffusion thermal transfer. It permits controllable lateral and vertical modulations of the dopant distributions of a polymer film for organic devices. Furthermore, it is the intention to combine the selectively doped (patterned) polymer layers with the vacuum deposited small-molecule films, such that both the heterostructure versatility of small-molecule materials and the patterning advantage of polymeric materials could be conserved in the hybrid polymer/small molecule heterostructure OLEDs. The rather thermally stable polymer poly(N-vinylcarbazole) (PVK), which is hole-transport and deep-blue emitting, in combination with several electron-transport molecules has been studied to demonstrate the feasibility of these concepts.

In this paper, we report that novel tetraphenyl- tetraalkynyl-COTs and octaaryl-COTs can be used as electron transport and hole blocker layer in organic LEDs. These materials have the similar reduction potentials compared to traditional electron transporters, such as Alq₃. The energy gaps for all of the COT derivatives studied here are very wide (> 3 eV), making it possible to fabricate blue emitting OLEDs with these materials. We have fabricated devices of the structure ITO/NPD400 angstrom/COT-Me 400 angstrom/Mg-Ag. (ITO equals indium-tin-oxide, NPD equals N,N'-diphenyl-N,N'-dinapthylbenzidine). The external quantum yield reaches 0.15% and the emission comes from NPD ((lambda) _max equals 436 nm). When 1% perylene (by weight) is doped into the NPD, the (lambda) _max of emission shifts to the 454 nm with the external QE reaches 0.60%. It indicates the efficient energy transfer from NPD to perylene. Changing the COT layer thickness from 400 Angstrom to 200 Angstrom, the emission is still from the NPD layer, as seen before. However, the I-V curve and turn-on voltage both shift to lower potentials by ca 2 - 3 V with the QE drops to 0.06%. The device QE can be increased by either using thin Alq layer or replacing Mg:Ag with LiF/Al electrode. Finally, the devices using COTs as hole blocker are fabricated and results are discussed.

We report on the fabrication and properties of single layer green light-emitting diodes (LEDs) based on fluorene- containing conjugated polymers and associated blends. We have used a new green fluorene based conjugated polymer (namely 5BTF8) as the emissive material as well as the host in blends with a guest hole transport triarylamine/fluorene copolymer (namely BFB) to fabricate bright and efficient single layer green polymer light-emitting diodes (PLEDs). An enhancement in both the electroluminescence quantum and power efficiency is seen for the blend. This observation indicates that the hole transport material leads to a significant improvement in hole injection and transport and thus to an improved charge carrier balance factor. A higher brightness and a lower turn on as well as operating voltage are also achieved for the blend. The emission from a green single layer LED with 5BTF8/BFB (4/1) as the emissive layer reaches a maximum brightness of 35000 cd/m² with a maximum external quantum efficiency of 1.3% or 4.2 cd/A and a maximum power efficiency of 2.5 lm/W. Novel small area LEDs were also fabricated using a SiN insulating layer on top of the ITO that allowed much higher brightnesses to be achieved compared to the standard area LEDs due to the reduced heating and therefor to a better thermal management of the device. The emission from a PEDOT/5BTF8 small area LED reached a maximum brightness of 155,000 and 6,500,000 cd/m² in DC and pulsed mode, respectively.

We propose a new cathode for organic light-emitting devices based on anodic porous alumina films. For this purpose usual and modified porous alumina and structured aluminum with removed porous alumina layer can be used. The usage of porous alumina cathode results in more inhomogeneous electric field and, as a consequence, more intensive auto electron emission and cathode efficiency. Owing to extended surface this matrix also gives the possibility to reach higher luminophor concentration without noticeable luminescence quenching. Organic electroluminescent devices of this type are easily produced and can be more efficient and stable as compared with usual layered structures. An intensive secondary electron emission is observed after irradiating of activated alumina matrix by an electron beam. We developed one of the proposed porous alumina cathode based organic electroluminescence structure doped with amorphous naphthalimide derivative. A bright electroluminescence of naphthalimide in such a structure was observed for the first time. The ways of the further development of this structure are pointed out.

A new organic material Ligand Lhn4 is used to fabricate organic light emitting diodes. There are two peaks in the Photoluminescence (PL) spectrum of Ligand Lhn4; they are 444 nm and 482 nm. Organic light emitting diode with Al/LiF(0.5 nm)/Ligand Lhn4(50nm)/NPB(50nm)/ITO structure was grown by evaporation. The electroluminescence (EL) spectrum shows there is one peak wavelength for OLED located at 486 nm. The full width of half maximum (FWHM) is about 112 nm, it mean the material Ligand Lhn4 is quite fair for fabrication of white light OLED. The OLED start emit a white near to blue color light at 2 V voltage, and the color have no change as the voltage increasing. The I-V and Brightness vs. Voltage curves were measured at air. The maximum brightness is 300cd/m² at voltage of 15 V. A doped device Al/LiF/BePP2:Ligand Lhn4(10%)/NPB/ITO is fabricated to improve the brightness and efficiency of OLED based on Ligand Lhn4.

The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications is probed by doping the blue light emitting methyl-substituted ladder-type poly(para-phenylene) with small concentrations of a highly fluorescent (pi) -conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence are modeled and discussed by means of a two step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Forster type mechanism alone cannot account for the experimental facts in such a guest host system. We have used the same materials to tune the emission color of organic- light-emitting diodes (OLEDs). Electroluminescence (EL)- current characteristics of the fabricated devices are presented. We found that the EEM process in OLEDs is more efficient for the EL process due to charge carrier trapping on the guest material compared to PL. For dopand concentrations as low as 1 w% one observes an increase of the photoluminescence as well as the electroluminescence quantum efficiency ((eta) _EL) by approximately 50% with respect to the pure host polymer efficiencies. We realized a white light emitting diode with an external (eta) _EL of up to 1.8% with the emission color being independent of the applied bias.

The synthesis of two families of electrically active and highly luminescent TPD-based copolymers is reported. (1) The Horner-olefination between TPD-dialdehydes and xylylene bisphosphonates has been used to prepare red and green emitting conjugated TPD-PPV copolymers 1 - 4. Here the TPD (triphenylamine dimer) moieties are brigded through alkoxy- substituted p-phenylene vinylene segments. (2) Blue emitting, nonconjugated TPD-xylylene copolymers (Poly-TPD- DPX 5, (6) have been synthesized by an electrophilic aralkylation using diphenylxylylene diol and TPD as the monomers. All these TPD-copolymers constitute amorphous electro-optical materials possessing remarkably high glass transition temperatures (Tg 110 - 240 degrees Celsius). Here we demonstrate strong lasing in the red, green and blue spectral region employing thin layers (approximately 100 nm) of these solution processable polymeric materials. In waveguiding neat films traveling-wave lasing (amplified spontaneous emission -- ASE) is achieved upon picosecond pulse excitation at 347 nm. Pump energy density thresholds as low as 3 (mu) J/cm² and ASE-line halfwidths approximately 10 nm have been observed. Comparable to the typical redox behavior of free TPD molecule the novel TPD- based polymers exhibit fully reversible electron transfer at low potential (E^Ox approximately 0.65 V), which is favorable for hole injection and stable charge transport in the semiconducting organic materials. In addition, these high-Tg polymers can act as the electro-active materials in LEDs, photovoltaic cells and photorefractive devices. The waveguiding properties of Poly-TPD-DPX were determined in planar and strip waveguides to be 12 dB/cm at 640 nm, and 2 dB/cm at 1550 nm.

In aromatic polyimides based on 9,10- bis(aminophenyl)anthracene or 9,10- bis(phenylthio)anthracene, charge carrier transport has been studied by conventional time-of-flight techniques. The electric field and temperature dependencies of both hole and electron drift mobility were observed. In amorphous film of the soluble polyimide, the drift mobility was found to reach the value of 10^-4cm²V^-1s^-1. In film of the insoluble polyimide containing a crystalline phase, the mobility was lower by one or two orders of magnitude. The result is attributed to the presence of cavities in the crystalline film. The applicability of known theoretical models describing the temperature and electric field dependencies of mobility is discussed. It was found out that J-aggregates, formed in the soluble polyimides doped with cyanine dye molecules, play an active role in charge carrier transport in the electroluminescence composites.

The steady state photoinduced absorption (PA), photoluminescence (PL), PL detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) N_O dependence of the PA, PL, and PLDMR signals are analyzed by means of a new rate equation model. It describes the dynamics of singlet excitons (SE's) and polarons in all three experiments with the same set of parameters. This yields the first quantitative analysis of the interaction of SE's and polarons in conjugated polymers. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin 1/2 magnetic resonance conditions reduce the total polaron and triplet exciton (TE) populations. Since the sublinear N_O dependence of the positive (PL-enhancing) spin 1/2 PLDMR and the polaron PA band are essentially the same, we conclude that that PLDMR is due to reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. It also suggests that it is a very significant process in luminescent (pi) - conjugated materials and organic light-emitting devices, which needs to be taken into account especially at high excitation densities such as in lasing action.

The absolute photoluminescence quantum yields ((Phi) _PL) of three end-capped oligothiophene derivatives dispersed in N,N'-((alpha) -naphthyl)-N,N'-diphenyl-1,1'-biphenyl ((alpha) -NPD) have been evaluated and the most efficient of the emitters was used as a dopant in molecular organic LEDs. Composite films of 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-furan (EC5FUR); 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-oxazole (EC5OXZ) and 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl)thien-2-yl]-1,3,4- oxadiazole (EC5OXD) doped into (alpha) -NPD were found to have (Phi) _PL values of 78, 62 and 28%, respectively. MOLED devices were fabricated using an EC5FUR/(alpha) -NPD composite as the emitting layer and the external quantum efficiencies ((eta) _EL) of these devices were evaluated. The results of the device characterization show that the inclusion of EC5FUR in the NPD hole transport layer increases the device (eta) _EL to 1.45% at a current density of 10 mA/cm². In addition, the concentration dependence of the (eta) _EL on the EC5FUR dopant in certain device structures when considered in conjunction with the current results of ultraviolet photoemission spectroscopic experiments suggests that this dopant species may be acting as both a hole and electron trap in the (alpha) -NPD host.

The present study focuses on the photoluminescence and electroluminescence properties of a new fluorescent red emitter based on a derivative of a polyaromatic hydrocarbon. 6,13-diphenylpentacene has a very narrow emission spectrum giving rise to a saturated red color with excellent chromaticity coordinates (x equals 0.64 and y equals 0.34) in accordance with the Commission Internationale de L'Eclairage (CIE). Efficient organic electroluminescent devices with saturated red emission were developed using a composite of 6,13-diphenylpentacene and Alq₃ as the active emissive layer. An electroluminescence quantum efficiency of 1.4% at 100 A/m², close to the theoretical limit (1.5%) (based on the measured fluorescence quantum yield, (phi) _pl equals 30%, of the emitting layer) was measured for an unoptimized device structure that consists of the emissive layer sandwiched between hole and electron transport layers. In addition, the quantum efficiencies of the pentacene-based devices are stable over a wide range of current densities (1 to 1000 A/m²).

Transparent and highly conducting zirconium-doped indium oxide (ZIO) thin films have been grown by pulsed laser deposition (PLD) on glass substrates without a post- deposition anneal. The structural, electrical and optical properties of these films were investigated as a function of film composition and substrate deposition temperature. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 1 J/cm² at growth temperatures ranging from 20 degrees Celsius to 400 degrees Celsius in oxygen pressure ranging from 1 mTorr to 25 mTorr. The films (approximately 2000 angstrom thick) deposited at 200 degrees Celsius in 25 mTorr of oxygen show electrical resistivities as low as 2.7 X 10^-4 (Omega) -cm, the average visible transmittance of 89%, the refractive index of 1.99 and optical band gap of 4.1 eV. These ZIO films were used as a transparent anode contact in organic light emitting diodes (OLEDs) and the device performance was studied. The external quantum efficiency measured at 100 A/m² for the [ZIO/TPD/Alq₃/MgAg] diodes was about 0.9%. Low driving voltage and high light emission were observed for the OLEDs with the ZIO anode.